1. Field of the Invention
This invention relates to the manufacture of graphite electrodes from petroleum coke, and more particularly to the inhibition of "puffing" of coke during graphitization of premium coke electrodes. These electrodes, primarily used in the electric furnace steel making process, must meet rigid specifications. Much of the petroleum coke presently available commercially has a tendency to expand rapidly and irreversibly during the heat treatment required as part of the electrode manufacturing process. This expansion is commonly referred to as "puffing". When the puffing phenomenon occurs to too great an extent, the electrode is rendered useless.
2. Prior Art
The reduction or elimination of the puffing phenomenon has been acomplished in the past by adding a puffing inhibitor to a blend of calcined petroleum coke and binder pitch prior to extruding and baking the electrode.
A method of controlling puffing comprising adding sodium carbonate prior to extruding the electrode is described in U.S. Pat. No. 2,814,076.
A method of controlling puffing by addition of iron oxide prior to extruding and baking an electrode is described in U.S. Pat. Nos. 3,506,745 and 3,624,231.
A method of blending petroleum coke with an oxygen-containing compound of iron, calcium, aluminum or manganese to inhibit puffing during graphitization is described in U.S. Pat. No. 3,842,165.
The addition of a titanium or zirconium compound with conventional puffing inhibitors is described in U.S. Pat. No. 3,563,705; and the addition of calcium cyanamid as a puffing inhibitor is disclosed in U.S. Pat. No. 3,642,962.
A method of producing a low sulfur coke comprising addition of an iron compound and a metal chloride, either to a coker feedstock or subsequent to the coking operation, is described in U.S. Pat. No. 3,873,427. The process described in that patent contemplates addition of a large amount, such as from 3 to 25 weight percent, of an iron compound along with a metal chloride, and the resulting low sulfur coke is not suitable for electrode manufacture because of the high level of impurities introduced during the desulfurization step.
Conventional commercial graphite electrode manufacture presently utilizes the addition of from 0.5 to 3.0 percent iron oxide mixed with coke and binder pitch prior to extrusion and baking of the electrode. This procedure has been quite effective in controlling puffing of the electrode during the graphitization step.
It is an object of the present invention to provide an improved process for reducing or eliminating puffing during manufacture of graphite electrodes.
It is a further object of the invention to provide such a process which results in an electrode having a lower level of impurities than results from conventional methods of manufacture.